U.S. patent number 6,161,923 [Application Number 09/121,258] was granted by the patent office on 2000-12-19 for fine detail photoresist barrier.
This patent grant is currently assigned to Hewlett-Packard Company. Invention is credited to Paul J. Benning, Diana D. Granger, Gerald T. Kraus, David Pidwerbecki, Joe E. Stout.
United States Patent |
6,161,923 |
Pidwerbecki , et
al. |
December 19, 2000 |
Fine detail photoresist barrier
Abstract
The photoresist barrier layer of an inkjet printer printhead is
processed to enable channels narrower than a predetermined width in
the barrier layer to be created without blockage. Relatively large
volumes of photoresist which form a wall of the channel are exposed
to a partial exposure of electomagnetic radiation to yield a
reduced concentration of photoresist barrier layer in the large
volume.
Inventors: |
Pidwerbecki; David (Corvallis,
OR), Kraus; Gerald T. (Albany, OR), Benning; Paul J.
(Albany, OR), Granger; Diana D. (Corvallis, OR), Stout;
Joe E. (Corvallis, OR) |
Assignee: |
Hewlett-Packard Company (Palo
Alto, CA)
|
Family
ID: |
22395529 |
Appl.
No.: |
09/121,258 |
Filed: |
July 22, 1998 |
Current U.S.
Class: |
347/63;
347/65 |
Current CPC
Class: |
B41J
2/1404 (20130101); B41J 2002/14387 (20130101); B41J
2002/14403 (20130101); B41J 2002/14467 (20130101); B41J
2202/03 (20130101) |
Current International
Class: |
B41J
2/14 (20060101); B41J 002/05 () |
Field of
Search: |
;347/63,65 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Buskirk, et al, "Development of a High-Resolution Thermal Inkjet
Printhead", Oct. 1998, Hewlett-Packard Journal, pp. 55-61. .
Askeland, et al, "The Second-Generation Thermal Inkjet Structure",
Aug. 1988, Hewlett-Packard Journal, pp. 28-31..
|
Primary Examiner: Barlow; John
Assistant Examiner: Stephens; Juanita
Attorney, Agent or Firm: Jenski; Raymond A.
Claims
We claim:
1. A printhead for an inkjet printer comprising:
an ink ejection mechanism disposed on a substrate;
an orifice plate having at least one orifice through which ink is
ejected by said ink ejection mechanism; and
a barrier layer comprising a photoresist material and disposed
between said substrate and said orifice plate, said barrier layer
patterned into features, a first feature and a second feature of
said features each including at least one exterior wall having
contact with ink flowing to said ink ejection mechanism and each
exterior wall at least partially encompassing a respective first
and second interior volume of said first feature and second feature
of said features, said first interior volume having a lesser
concentration of said photoresist material than said second
interior volume.
2. A printhead in accordance with claim 1 wherein said first
feature of said features is disposed spaced apart from said second
feature of said features by a distance less than a predetermined
distance.
3. A printhead in accordance with claim 2 wherein said
predetermined distance is in the range of 13 .mu.m to 20.5
.mu.m.
4. A printhead in accordance with claim 2 wherein said barrier
layer further comprises said barrier layer having a predetermined
thickness dimension and wherein said predetermined distance is less
than a multiple of 1.5 of said predetermined thickness
dimension.
5. A printhead in accordance with claim 1 wherein said photoresist
material further comprises a free radical polymerized
photoresist.
6. An inkjet print cartridge comprising:
a housing for containing a supply of ink; and
a printhead affixed to said housing and adapted to accept ink from
said supply of ink, said printhead further comprising:
an ink ejection mechanism disposed on a substrate,
an orifice plate having at least one orifice through which ink is
ejected by said ink ejection mechanism, and
a barrier layer comprising a photoresist material and disposed
between said substrate and said orifice plate, said barrier layer
patterned into features, a first feature and a second feature of
said features each including at least one exterior wall having
contact with ink flowing to said ink ejection mechanism and each
exterior wall at least partially encompassing a respective first
and second interior volume of said first feature and second
feature, said first interior volume having a lesser concentration
of said photoresist material than said second interior volume.
7. An inkjet cartridge in accordance with claim 6 wherein said
first feature of said features is disposed spaced apart from said
second feature of said features by a distance less than a
predetermined distance.
8. An inkjet cartridge in accordance with claim 7 wherein said
predetermined distance is in the range of 13 .mu.m to 20.5
.mu.m.
9. An inkjet cartridge in accordance with claim 7 wherein said
barrier layer further comprises said barrier layer having a
predetermined thickness dimension and wherein said predetermined
distance is less than a multiple of 1.5 of said predetermined
thickness dimension.
10. An inkjet cartridge in accordance with claim 6 wherein said
photoresist material further comprises a free radical polymerized
photoresist.
11. An inkjet printer comprising:
a motor for positioning a print cartridge relative to a medium to
be printed upon; and
print cartridge further comprising:
a housing for containing a supply of ink, and
a printhead affixed to said housing and adapted to accept ink from
said supply of ink, said printhead including an ink ejection
mechanism disposed on a substrate to selectively deposit ink onto
said medium, an orifice plate having at least one orifice through
which ink is ejected by said ink ejection mechanism, and a barrier
layer comprising a photoresist material and disposed between said
substrate and said orifice plate, said barrier layer patterned into
features, a first feature and a second feature of said features
each including at least one exterior wall having contact with ink
flowing to said ink ejection mechanism and each exterior wall at
least partially encompassing a respective first and second interior
volume of said first feature and said second feature of said
features, said first interior volume having a lesser concentration
of said photoresist material than said second interior volume.
12. An inkjet printer in accordance with claim 11 wherein said
first feature of said features is disposed spaced apart from said
second feature of said features by a distance less than a
predetermined distance.
13. An inkjet printer in accordance with claim 12 wherein said
predetermined distance is in the range of 13 .mu.m to 20.5
.mu.m.
14. An inkjet printer in accordance with claim 12 wherein said
barrier layer further comprises said barrier layer having a
predetermined thickness dimension and wherein said predetermined
distance is less than a multiple of 1.5 of said predetermined
thickness dimension.
15. An inkjet printer in accordance with claim 11 wherein said
photoresist material further comprises a free radical polymerized
photoresist.
Description
BACKGROUND OF THE INVENTION
The present invention is generally related to a photoresist barrier
layer capable of reproducing fine details and more particularly
related to a barrier layer in an inkjet printer printhead that
utilizes small dimensions to produce reduced drop weight ink
drops.
Inkjet printers operate by expelling a small volume of ink through
a plurality of small orifices in an orifice plate held in proximity
to a medium upon which printing or recording marks are to be
placed. These orifices are arranged in a fashion in the orifice
plate such that the expulsion of drops of ink from a selected
number of orifices relative to a particular position of the medium
results in the production of a portion of a desired character or
image. Controlled repositioning of the orifice plate or the medium
followed by another expulsion of ink drops results in the creation
of more segments of the desired character or image. Furthermore,
inks of various colors may be coupled to individual arrangements of
orifices so that selected firing of the orifices can produce a
multicolored image by the inkjet printer.
Several mechanisms have been employed to create the force necessary
to expel an ink drop from a printhead, among which are thermal,
piezoelectric, and electrostatic mechanisms. While the following
specification is made with reference to a thermal ink ejection
mechanism, the present invention may have application for the other
ink ejection mechanisms as well.
Expulsion of the ink drop in a conventional thermal inkjet printer
is a result of rapid thermal heating of the ink to a temperature
that exceeds the boiling point of the ink vehicle to create a vapor
phase bubble of ink. Such rapid heating of the ink is generally
achieved by passing a pulse of electric current through an ink
ejector that usually is an individually addressable heater
resistor, typically for 1 to 3 microseconds, and the heat generated
thereby is coupled to a small volume of ink held in an enclosed
area associated with the heater resistor and that is generally
referred to as a firing chamber. For a printhead, there are a
plurality of heater resistors and associated firing
chambers--perhaps numbering in the hundreds--each of which can be
uniquely addressed and caused to eject ink upon command by the
printer. The heater resistors are deposited in a semiconductor
substrate and are electrically connected to external circuitry by
way of metalization deposited on the semiconductor substrate.
Further, the heater resistors and metalization may be protected
from chemical attack and mechanical abrasion by one or more layers
of passivation. Additional description of basic printhead structure
may be found in "The Second-Generation Thermal InkJet Structure" by
Ronald Askeland et al. in The Hewlett-Packard Journal, August 1988,
pp. 28-31. Thus, one of the walls of each firing chamber consists
of the semiconductor substrate (and typically one firing resistor).
Another of the walls of the firing chamber, disposed opposite the
semiconductor substrate in one common implementation, is formed by
the orifice plate. Generally, each of the orifices in this orifice
plate is arranged in relation to a heater resistor in a manner that
enables ink to be expelled from the orifice. As the ink vapor
bubble nucleates at the surface of the heater resistor and expands,
it displaces a volume of ink that forces a volume of ink out of the
orifice for deposition on the medium. The bubble then collapses and
the displaced volume of ink is replenished from a larger ink
reservoir by way of an ink feed channel in another of the walls of
the firing chamber.
As users of inkjet printers have begun to desire finer detail in
the printed output from a printer--especially in color output--the
technology has been pushed into smaller drops of ink to achieve the
finer detail. Smaller ink drops means lowered drop weight and
lowered drop volume. Production of such low drop weight ink drops
requires smaller structures in the printhead. Thus, smaller firing
chambers (containing a smaller volume of ink), smaller ink
ejectors, and smaller orifice bore diameters are required.
A majority of the size of the firing chamber is determined by a
layer of photoimagable polymer sandwiched between the heater
resistor-bearing semiconductor substrate and the orifice plate.
This layer is traditionally known as a barrier layer and has often
been described, see for example, "Development of High-Resolution
Thermal Inkjet Printhead", by William A. Buskirk, et al., the
Hewlett-Packard Journal, October 1988, pp. 55-61. The barrier layer
is honeycombed with cavities that, when bounded by the substrate on
one side and by the orifice plate on the other, become the ink
firing chambers and connecting inkfeed channels that route ink into
the ink firing chambers. The dimensions and architecture of the ink
firing chambers, the ink feed channels, and other features which
control and filter ink are typically defined and created by
photoimaging techniques. These techniques are capable of creating
relatively small features in the barrier material.
A problem that occasionally manifest itself in inkjet printheads is
that of occlusion or narrowing occurring in an ink feed channel or
in the orifice of the printhead. Microscopic particles can become
lodged in the channel leading to the ink firing chamber, causing
premature failure of the heater resistor, misdirection of ink
drops, or diminished ink supply to the firing chamber resulting in
greatly diminished ink drop size. A single orifice, which does not
fire an ink drop when it is commanded to do so, leaves a missing
portion from a printed character or creates a band of missing drops
from a printed image. The end result is perceived as a poorer
quality of printed matter, a highly undesirable characteristic for
an inkjet printer. To resolve this undesirable result, others have
used spare or redundant orifices to eject ink, multiple inlets to
the ink firing chamber, and pillars or islands formed in the
barrier layer and disposed in the ink path to filter particles from
the ink.
As the size of the firing chamber, ink feed channels, and filtering
features become smaller--for example approximately the same
dimensions as the thickness of the barrier layer--the conventional
barrier layer photoimaging process is unable to resolve the finer
details of the desired architecture. This inability places a
limitation on the smallest size of the architectural features and
will limit the reliability of the ejection of ink when the smallest
features are not properly formed.
SUMMARY OF THE INVENTION
An inkjet printhead having fine details in a barrier layer includes
a photoresist layered to a predetermined thickness on a substrate.
A first volume a second volume, and a third volume of the
photoresist are selected to remain after development of the
photoresist. When the first volume and the second volume are spaced
apart by a distance less than a predetermined distance, the first
volume is exposed to less than a full exposure of electromagnetic
radiation. The third volume of the photoresist is exposed to a full
exposure of electromagnetic radiation. The exposed photoresist is
then developed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified diagram of a printer which may employ the
present invention.
FIG. 2 is an isometric view of a print cartridge which may be used
in the printer of FIG. 1 and which may employ the present
invention.
FIG. 3 is a cross sectional elevation view of the printhead
illustrating, in particular, ink ejection chambers and which may be
employed in the inkjet print cartridge of FIG. 2. This cross
section is taken at A--A of FIG. 2.
FIG. 4 is an isometric plan view of the barrier layer and substrate
of a printhead which may employ the present invention.
FIG. 5A and 5B are cross sectional elevation views of an ink feed
channel which illustrate the undesirable effect of barrier layer
bridging.
FIG. 6 is an isometric plan view of the barrier layer and substrate
of a printhead, illustrating a first alternative embodiment which
may employ the present invention.
FIG. 7 is an isometric plan view of the barrier layer and substrate
of a printhead, illustrating a second alternative embodiment which
may employ the present invention.
FIGS. 8A and 8B is a greatly enlarged portion of a cross section
(for example through B--B of FIG. 7) of a printhead substrate and
barrier material during (FIG. 8A) and after (FIG. 8B) exposure to
electromagnetic energy.
FIG. 9 is an example of a mask which may be employed in the present
invention.
FIG. 10 is a flowchart of the process which may be employed to
produce the barrier layer employed in the present invention
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention encompasses photoresist details that are
added to the architecture-defining features of the photoresist so
that the resolution of the details of the features is enhanced. As
previously indicated, this improvement in resolution, or fine
detail definability, has particular application in an inkjet
printhead, but may be equally well used in other applications such
as printed circuit board electrical trace definition.
A simplified diagram of an inkjet printer which may use the present
invention is shown in FIG. 1. A medium to be printed upon 101 is
moved past one or more print cartridges 103 and 105 in a direction
out of the plane of the figure of FIG. 1 by a platen motor 109. Two
print cartridges are shown for generality but it is recognized that
a particular printer may employ only a single print cartridge (for
example, for black ink printing) or many print cartridges (for
example, for multicolor printing). The print cartridges 103 and 105
are mounted in a cartridge carrier 107 and are scanned in
conventional fashion back and forth across the medium 101 in an
orthogonal, or scan, direction by a carriage motor 111. The platen
motor 109 and the carriage motor 111 are typically under the
control of a media and cartridge position controller 113. An
example of such positioning and control apparatus may be found
described in U.S. Pat. No. 5,070,410. Thus, the medium 101 is
positioned in a location so that the print cartridges 103 and 105
may eject droplets of ink as required by the data that is input to
a drop firing controller 115 of the printer in a band parallel to
the scan direction as the print cartridges 103 and 105 are
translated across the medium by the carriage motor 111. When the
print cartridge 103 and 105 reach the end of their travel at an
edge of the medium 101, the medium is typically incrementally
advanced by a media position controller 113 and the platen motor
109, and the print cartridges 103 and 105 are returned along the
"X" axis.
An example of an inkjet cartridge that mechanically mounts in the
cartridge carrier 107 and is electrically coupled to the drop
firing controller 115 of the inkjet printer is shown in FIG. 2. A
cartridge body housing 201 houses a supply of ink and routes the
ink to a printhead 203 via ink conduits. Visible at the outer
surface of the printhead are a plurality of orifices 205 through
which ink is selectively expelled upon commands of the printer drop
firing controller, which commands are communicated to the printhead
203 through electrical connections 207 and associated conductive
traces (not shown). In one implementation of an inkjet print
cartridge, the printhead is constructed from a semiconductor
substrate, including thin film heater resistors disposed on or in
the substrate, the photoresist barrier layer, and the foraminous
orifice plate with, for example, orifice 205. Electrical
connections are typically made to the printhead 203 of the print
cartridge by way of a flexible polymer tape 209. Copper or other
conductive traces are deposited or otherwise secured on one side of
the tape so that electrical interconnections 207 can be contacted
with the printer and routed to the substrate. The tape can be bent
around an edge of the print cartridge, as shown, and secured.
A cross section of the printhead is shown in FIG. 3 and is taken
from part of the section A--A shown in FIG. 2. A portion of the
body 301 of the cartridge body housing 201 is shown where it is
secured to the printhead by an adhesive (in association with
pressure). In the preferred embodiment, ink is supplied to the
printhead by way of a common ink plenum 305 and through a slot 306
in the printhead substrate 307. (Alternatively, the ink may be
supplied along the sides of the substrate). Heater resistors and
their associated orifices are conventionally arranged in two
essentially parallel rows near the inlet of ink from the ink
plenum. In many instances the heater resistors and orifices are
arranged in a staggered configuration in each row and, in the
preferred embodiment, the heater resistors are located on opposite
sides of the slot 306 of the substrate 307, as exemplified by
heater resistors 309 and 311 in FIG. 3.
The orifice plate 303 in the preferred embodiment is produced by
electrodepositing nickel on a mandrel having pegs and dikes with
appropriate dimensions and suitable draft angles in the form of a
complement of the features desired in the orifice plate. Upon
completion of a predetermined amount of time in an
electrodeposition bath, a thickness of nickel will be deposited on
the mandrel. The resultant nickel film is removed after cooling and
mechanically planarized and treated for subsequent use. The nickel
orifice plate is then conventionally coated with a precious metal
such as gold, paladium, or rhodium to resist corrosion. Following
its fabrication, the orifice plate is affixed to the semiconductor
substrate 307 with the barrier layer 313, which also functions as
an adhesive. The orifices created by the electrodeposition on the
mandrel extend from the outside surface of the orifice plate 209
through the material to the inside surface, the surface that forms
one of the walls of the ink firing chamber. Usually, an orifice is
aligned directly over the heater resistor so that ink may be
expelled from the orifice without a trajectory error introduced by
an offset.
The substrate 307 and orifice plate 303 are affixed together by the
barrier layer material 313. In the preferred embodiment, the
barrier layer material 313 is disposed on the substrate 307 in a
patterned formation such that firing chambers 315 and 317 are
created in areas around the heater resistors. The barrier layer
material is also patterned so that ink is supplied independently to
the firing chambers by one or more ink feed channels. Ink drops 319
are selectively ejected upon the rapid heating of a heater resistor
upon command by the printer. The substrate having the barrier layer
affixed to one surface is then positioned with respect to the
orifice plate such that the orifices are precisely aligned with the
heater resistors of the substrate.
The barrier layer 313, in the preferred embodiment, utilizes a
negative dry film photoresist in which polymethyl methacrylate
(PMMA) or similar materials that are polymerized in a free radical
reaction and withstand relatively aggressive fluids like ink.
Examples may be found in European Patent Application No. EP 0 691
206 A2 "Ink Jet Printhead Photoresist Layer Having Improved
Adhesion Characteristics" published Jan. 10, 1996. In the preferred
embodiment, the barrier layer is first applied as a continuous
layer upon the substrate 307 with the application of sufficient
pressure and heat suitable for the particular material selected.
Generally, the barrier layer film is sandwiched between thin
protective sheets of material prior to its use in a printhead. One
sheet is removed to enable lamination of the barrier layer to the
substrate. The other sheet is left in place until after the barrier
layer is exposed. The photoresist barrier layer is exposed through
a negative mask to ultraviolet light (preferably in the range of
wavelengths of 440-340 nm, I-line) to polymerize the barrier layer
material. The protective film sheet is removed and the exposed
barrier layer, in the preferred embodiment, is subjected to a
chemical wash using a developer solvent of a 74:26 w/w % mixture of
N-methyl-2-pyrrolidone and diethylene glycol so that the unexposed
areas of the barrier layer are removed by dissolution. The external
walls of the remaining areas of barrier layer form the walls of
each ink firing chamber around each heater resistor. Also, the
remaining areas of barrier layer form the walls of ink feed
channels that lead from the ink firing chamber to a source of ink
(such as the ink plenum 305 by way of the slot as shown in FIG. 3)
as well as filtering features in the ink path. The ink feed
channels enable the initial fill of the ink firing chamber with ink
and provide a continuous refill of the firing chamber after each
expulsion of ink from the chamber. The rate at which ink can enter
and fill the ink firing chamber is a significant factor in
determining the highest speed at which the printer can print. In
the preferred embodiment, two ink feed channels are created in the
barrier layer to couple the ink plenum to the ink firing chamber so
that a redundant supply of ink is maintained to the chamber and
that a high rate of refill can be realized.
One additional feature is created in the barrier layer of the
preferred embodiment. At the entrance to each ink feed channel
there is disposed a plurality of outer barrier layer islands 401
such as shown in the isometric plan view of the surface of the
substrate (with the orifice plate removed) of FIG. 4. Each outer
barrier island is composed of barrier material and extends the full
thickness of the barrier layer 313 from the substrate 307 to the
orifice plate. In order to avoid delamination of the islands from
either the orifice plate or the substrate, each outer barrier
island offers an area of adhesion of approximately the square of
the barrier thickness to each surface. The major purpose of these
outer barrier islands is to prevent particles and contaminants from
the ink from reaching the ink feed channels and the orifice of each
firing chamber. In order to function properly, this filter requires
that the spaces (S) between each island (the equivalent of filter
pores) be smaller than the channel width (W) of each firing chamber
and smaller than the diameter of the orifice bore. Thus, any
contaminant that could lodge in the ink feed channel or in the
orifice is blocked from these critical areas. As a result of a
number of islands (and spaces between), the blockage of any one of
the spaces between the islands does not seriously impede the flow
of ink to each ink feed channel and the likelihood of occlusion of
an ink firing chamber is considerably reduced.
In the preferred embodiment, the dimensions of many of the elements
of the printhead have been made significantly smaller than
previously known designs to produce a high quality of ink printing
by using small ink drops. The nominal ink drop weight is
approximately 10 ng for ejection from an orifice having a bore
diameter of 18 .mu.m. In order to achieve an ink firing chamber
refill rate supportive of a 15 KHz frequency of operation, two
offset ink feed channels 403, 405 are employed to provide redundant
ink refill capability. Each ink feed channel has a channel width W
of 18.5 .mu.m and a channel length of approximately 30 .mu.m.
Channels and orifices of these dimensions present a greater
challenge to the filtering of contaminants than previously
undertaken in that particles the size of human skin cells will
block an ink feed channel or orifice. Since particles of this size
include some biological cells that are non-rigid, the filter pore
size must be less than the smallest operational dimension of the
printhead to trap the potentially blocking particle. Depending upon
the particular application, the smallest operational dimension is
either the ink feed channel, W, of 18.5 .mu.m or the orifice bore
diameter of approximately 18 .mu.m. In the preferred embodiment,
the spacing (S) between each island is 12 .mu.m. The thickness, T,
of the barrier layer is 14 .mu.m.
Negative photoresists are well-known for resolution limitations
primarily due to swelling during the material photo development
process. It is known that any feature defined in the barrier layer,
or the space between any such feature, should have dimensions that
exceed the thickness dimension of the barrier layer. See, Weiss,
"Photoresist Technology Update", Semiconductor International, April
1983. Weiss states that negative photoresist materials are limited
to layer thickness to feature dimensions of 1:2 or 1:3 ratios while
positive resists were capable of 1:1 ratios. The typical procedures
for insuring manufacturable designs are to maintain photoresist
features exceeding these minimum aspect ratios. If these guidelines
are followed, generally, the photoresist features can be resolved,
developed, and adherence to the thinfilm structure maintained
within the expected manufacturing variation of photoresist material
quality and tool variation. In the development of low drop weight
ink drop cartridges, however, it was found that the physical limits
of being able to resolve features while still adhering to the
manufacturability guidelines would not always apply to some
features. Specifically, small, fine features in the barrier
material that were surrounded by voids of material were easily able
to be produced without bridging. Similar features that were not
surrounded by voids experienced significant bridging problems.
An example of a desired ink feed channel cross section for a low
drop weight cartridge is shown in FIG. 5A. The substrate 307 has
the barrier layer 313 disposed on its surface. Orifice plate 209 is
secured to the barrier layer 313. The barrier layer has had a
channel 501 photodefined and developed into the barrier layer so
that an ink feed channel has been created by the sandwich of
substrate, barrier layer, and orifice plate. Consistent with
expectations derived from Weiss, the ink feed channel, having a
barrier thickness to feature dimension, W, of 1:1.3, would develop
incompletely. This incomplete development results in a bridge 503
of barrier layer remaining across the narrow channel as shown in
FIG. 5B. This bridge occludes the channel and reduces the volume of
ink flow to the ink firing chamber. Curiously, the spacing between
the islands, S, did not exhibit the same bridging even though the
barrier thickness to feature dimension, S, was 1:0.9.
It is believed that the depletion of polymerization inhibitor, e.g.
oxygen, during exposure limits the feature dimension (channel
width) that can be defined between large volumes of negative
photoresist. For a given barrier thickness, exposure dose, dose
rate, temperature, and oxygen availability at the barrier surface,
inhibitor diffusion is believed to be limited to a finite distance.
When barrier thickness is such that a channel is defined within
this distance, the inhibitor diffusion proximity effect becomes
more important than swelling in limiting aspect ratio.
When an area of barrier layer material is exposed to non-ionizing
radiation such as ultraviolet (UV) electromagnetic radiation,
chemical reactions are induced in the barrier film that form free
radicals. These free radicals initiate crosslinking chain reactions
that make exposed areas immune to the developer solvent and thus
define the desired image; however, in a usual manufacturing
environment, molecular oxygen from the air is in equilibrium with
the other components in the barrier layer film. Before the
cross-linking reactions may ensue, the oxygen molecules--which are
much more reactive to free radicals--must first be consumed. Once
the concentration of radicals required to react with the
immediately-available oxygen and other polymerization inhibitors is
exceeded, further radiation cross-links the material.
The proximity effect that caused "incomplete development" (or
"bridging") occurs at the interface between the exposed and
unexposed volumes of barrier material: the exposed side has been
depleted of free oxygen molecules; the unexposed side still has the
equilibrium concentration. Thus, because the barrier layer is
separated from the oxygen in the air by its protective cover film
during manufacture of the printhead, a concentration gradient
forces migration of free oxygen molecules into the exposed volume
from the adjacent unexposed barrier material in order to equalize
the distribution of free molecular oxygen. Oxygen migration out of
the unexposed barrier material that will form the ink feed channel
then lowers the amount of radiation required to initiate a chain
reaction because there are fewer oxygen molecules to consume before
the onset of crosslinking, thus allowing the masked channel to be
undesirably exposed by radiation scattered from the unmasked
area.
An inkjet printhead employing the present invention is able to
reliably produce features in negative photoresist that are below
the 1:2 ratio limitation defined by Weiss. In a preferred
embodiment, the ratio of barrier layer thickness to feature
dimension is reliably produced at a ratio of 1:1.3 without bridging
even when the feature such as an ink feed channel is proximate a
large volume of exposed barrier material. Experiments have
indicated that ratios as low as 1:0.9 can be reliably produced. As
described above, the ink feed channel of a preferred embodiment
(formed by the wall of a large volume of barrier material and the
wall of an inner barrier island) has a width, W, of approximately
17 .mu.m using a barrier material with a thickness, T, of
approximately 14 .mu.m. That is, when an inner barrier island 605
is spaced apart from a large volume of barrier material 601, 603 to
create an ink feed channel 403 or 405, the large volume of barrier
material is partially exposed to electromagnetic energy to reduce
or eliminate bridging. Further in the preferred embodiment, the
outer barrier islands 401 are spaced apart from each other by a
distance (S) of approximately 10 .mu.m and spaced apart from the
nearest large volume of exposed barrier material by a distance (D)
of approximately 20 .mu.m. It should be noted that the dimensions
for the barrier layer features are given as the dimensions of the
photoresist mask. The spacings between barrier layer walls,
spacings such as S, the barrier island spacing, and W, the ink feed
channel width, are expected to become between 1 and 2 .mu.m larger
than the photoresist mask dimensions after the developing
process.
Several embodiments were created in pursuit of the discovery that
fine feature details that were smaller than the design guidelines
could be realized in areas where voids in the barrier material were
produced (such as the large voids surrounding the outer barrier
islands 401 and 403, for example). Two alternative embodiments are
disclosed herein that take advantage of the discovery. A first
alternative embodiment is shown in FIG. 6. Here, a portion of the
printhead substrate and the barrier layer material layered on top
of the printhead substrate is shown with the orifice plate removed.
A heater resistor 309 is supplied ink from two ink feed channels
403 and 405, which in turn draw ink from an ink feed slot 306 past
outer barrier islands 401. Neighboring firing chambers and
associated ink firing resistors and ink feed channels are separated
by peninsulas 601 and 603 in the barrier layer material. As in
previous designs, the ink feed channels 403 and 405 are defined by
exterior walls of the barrier material, either as part of the
peninsulas 601 and 603 or inner barrier island 605. The ink feed
channels, since they are disposed close to the peninsulas and a
large mass of barrier material, are subject to the aforementioned
bridging and plugging by oxygen-starved barrier material. In the
first alternative of the preferred embodiment, voids 607 and 609,
for example, are formed in the barrier material leaving a barrier
rim wall 611 having a thickness Q. In this embodiment, Q ranges
between 2 to 10 microns. The resulting barrier layer in the
interior volume of the peninsula areas, in products using the
present invention, exhibit a lower overall concentration of
photoresist material than solid volumes such as the barrier
islands.
A second alternative embodiment is shown in FIG. 7. Like FIG. 6,
FIG. 7 shows a top surface view of the wafer and barrier material
with the orifice plate removed. Rather than providing void volumes
in select portions of the barrier layer material, a portion 701 of
the barrier layer material is subjected to a partial exposure to
ultraviolet radiation (rather than a complete masking in a negative
photoresist material) so that small volumes of the otherwise masked
barrier material can donate oxygen molecules to those volumes that
are exposed to the full exposure of ultraviolet electromagnetic
radiation (so they will pull fewer molecules from the unexposed ink
feed channels 403 and 405). The effect of partial ultraviolet
electromagnetic radiation exposure can be appreciated by viewing
FIGS. 8A and 8B. A greatly exaggerated cross-sectional view of the
substrate 307 and the barrier material 313 is pictured in FIG. 8A.
A portion of the barrier material defining mask 801 includes an
ultra violet (UV) transparent layer 803 and a chrome UV opaque
layer 805. Conventionally, the barrier layer of the peninsulas and
other areas that are to be retained after developing are exposed to
a full exposure of ultraviolet radiation. In FIG. 8A, certain
segments are subdivided from the otherwise exposed area and are
prevented from such exposure by the chrome trace 805 of the mask
801. Accordingly, a segment 807 is shadowed by the mask and is
unexposed to the ultraviolet radiation. In the preferred embodiment
the chrome trace placed on the mask 801 is selected to be a square
of approximately 1 to 2 micrometers on a side. In practice, this
square produces an oxygen donating area which, because of its size,
itself becomes polymerized, thereby leaving only a small pit 809 at
the surface of the barrier material. In the second alternative
embodiment, the surface of the portion of the barrier material that
is selected for the oxygen-donating partial UV exposure takes on
the appearance of a shallowly pitted surface with a pseudo-random
distribution of pits. This is illustrated in FIG. 7. The interior
volumes of those areas partially exposed to electromagnetic
radiation, as described above, exhibit a lower concentration of
photoresist material (due to the remaining pits) than those areas
that are fully exposed. (Once the orifice plate is placed on the
barrier material and the barrier material is heated and compressed
in order to secure the orifice plate, most of the pits are
obliterated by the heat and compression).
A part of the mask that may be employed in producing the structured
barrier layer is shown in FIG. 9. The mask shown is for a negative
photoresist (black areas indicate no chrome traces) and encompasses
a single firing chamber and the two ink feed channels that lead to
the firing chamber. This part of the mask is reproduced many times
over in a conventional step-and-repeat process so that many firing
chambers can be created. In the second alternative embodiment, a
large number of chrome squares are disposed in a pseudo-random
pattern throughout the exposed regions near the narrow channels in
order to provide a partial masking of that portion of the barrier
material that is to be a molecular oxygen donor to those areas of
the barrier material which will receive full ultraviolet light
exposure. The fully exposed areas will draw fewer oxygen molecules
from unexposed areas that will be subsequently removed by the
development process. The lack of chrome deposition on the mask
corresponds to areas of the barrier material that will receive a
full exposure of ultraviolet radiation. In the preferred
embodiment, between fifteen and fifty percent-perferrably
twenty-five percent-of the exposure area is shadowed by chrome
squares. The pseudo-random pattern is established by first
subdividing the portion of the barrier material to be partially
exposed into groups of segments, four segments to a group, and
selecting one of the four segments to be shadowed by a chrome
square. A supergroup of 64 groups have one of the four squares
within the group selected for a chrome square on a true random
basis. The supergroup is then replicated throughout the portion of
the barrier material in order to provide the twenty-five percent
masking capability.
The process to produce the improved printhead is shown in FIG. 10.
The basic overall architecture of the barrier layer is first
established, step 1001, according to the drop volume and other
parameters desired for the entire printhead. A determination is
then made at 1003 for which features of the barrier layer
architecture are disposed closer together than the negative
photoresist conventional guidelines and, do not have nearby and
necessary voids in the barrier material such as, for example, the
outer barrier islands 401. As a result of this step there are two
types of features that are defined: those which require full UV
exposure and those which require a reduced UV exposure. Since the
photoresist in the preferred embodiment is a negative photoresist,
this is accomplished by depositing solid chrome on a UV transparent
mask (as described above) in areas that are to be shaded from the
electromagnetic radiation and depositing a micropattern of chrome
and no-chrome in areas that are to receive a reduced electomagnetic
radiation exposure. Thus, chrome is deposited (step 1005) on the
mask in areas where barrier material is to be removed.
Those areas of barrier material that are selected to be molecular
oxygen donors (and receive a reduced exposure to electromagnetic
radiation) are subdivided into segments at step 1007. A
determination is made regarding the desired amount of ultraviolet
radiation exposure the donor portions of the barrier substrate is
to receive (in the second alternative embodiment, this level is a
seventy-five percent exposure-a twenty-five percent masking). From
this determination of the desired exposure level, a proportion of
the segments is established, at step 1011, such that the proportion
of segments to be exposed relative to those segments not to be
exposed is set approximately equal to the desired exposure level of
step 1009. When considered in relation to the whole area to receive
a reduced exposure, the partial masking of the segments results in
a summation or integration of the exposure over the whole reduced
exposure area corresponding to the proportion of the unmasked
segments to masked segments. Thus, the oxygen molecule donor area
is subdivided into a multiplicity of segments. Groups of these
segments, for example a group of four contiguous segments, are
picked and one of the four segments of the group is randomly masked
with the chrome masked material as in step 1013. Chrome is then
deposited at the predetermined random or pseudo-random segments in
step 1005. A mask thereby results in which the photoresist material
is shadowed from ultraviolet electromagnetic radiation and volumes
of non-resistant barrier material established.
A semiconductor substrate having the appropriate thin film
processing completed to produce the firing resistors, electrical
interconnect, and any other required active or passive electronic
components is then further processed to add a photoresist material
on the same surface of the substrate as the thin film processing.
This photoresist is deposited in a conventional manner such as
lamination at step 1017. The mask is then aligned and the substrate
with the deposited photoresist is subject in step 1019 to I-line
ultraviolet electromagnetic radiation. Following exposure, the mask
is removed and the photoresist is subjected to a conventional
development process such as that described earlier and indicated at
block 1021.
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